1. Field of the Invention
This invention relates to compositions of a polyphenylene ether resin and a rubber modified, high impact polystyrene (HIPS) and, more specifically, to such blends in which the swell index of the HIPS rubber phase is regulated to control the surface gloss of the blend after molding.
2. Description of the Prior Art
For some time now, mixtures of polyphenylene ether resin and rubber modified, high impact polystyrene resin have enjoyed increasingly wider commercial use in industry, the household and in personal care products. Plastic items molded from such compositions are durable and exhibit additional properties that make them good substitutes for articles previously manufactured from more conventional materials such as metal, wood and ceramics. Because of the variety of applications, the compositions are usually formulated according to specific requirements. For instance, in some cases customer needs dictate that the product exhibit a matte, or relatively dull, finish. In still other cases a glossy or very shiny finish may be desired.
Manufacturing techniques have been developed to regulate the surface gloss of such molded products with reasonable accuracy. For instance, it is known that the surface gloss of molded parts can easily be lowered by increasing the size of the HIPS rubber particles used in the composition. Reducing the mold temperature during the injection molding cycle is another way of achieving this same result. However, both techniques have their shortcomings. The use of larger particle size rubber usually results in significant decreases in the notched Izod impact strength of the molded part. On the other hand, the use of lower mold temperatures can and often does markedly reduce the stress cracking resistance. Both properties are important and relate directly to the durability and toughness of the molded part.